It is known to cool the cooling water from a steam-powered machine, e.g. a condensate from a condensor receiving depleted steam therefrom or water circulated through a condensor associated with the steam cycle, in a natural draft cooling tower by causing the water to be cooled to trickle over baffles provided generally at a lower portion of the upright tower above an air inlet thereto.
The shape of the tower is that of a nozzle, e.g. a venturi nozzle with a progressive and curvilinear reduction in cross section upwardly to an outlet end disposed well above a sump below the air inlet and in which the cooled water collects.
The water to be cooled thus flows countercurrent to or in counterflow to the rising current or flow of cooling air which is accelerated upwardly by heating from the water to be cooled and the constricted flow cross section.
It is known, moreover, to utilize such a tower to discharge flue gases forming a combustion system, e.g. a boiler, into the atmosphere. In this case a gas outlet pipe, receiving the flue gas, can open above the heat exchanger zone provided with the baffles into the rising air stream.
In recent years the legal limits for solids, toxic and noxious contaminants in the flue gases from large scale combustion installations such as power plant boilers have been greatly reduced to the point where the flue gas must be subjected to an intensive gas cleaning operation.
Part of this gas cleaning operation, as a rule, requires a wet scrubbing or washing of the flue gas in conjunction with the addition of absorbents serving as a desulfurization medium.
The cleaned flue gas generally leaves the desulfurization apparatus at a temperature of 40.degree. to 60.degree. C. and must, if it is to be discharged by a conventional smoke stack, be heated at a temperature of about 80.degree. to 100.degree. C. to provide the requisite draft in the stack.
Naturally, this reheating has significant energy requirements. For example, in the case of a 70 MW power plant operating at full capacity, more than 2.5 million cubic meters of flue gas per hour must be reheated to the requisite temperature for discharge at the smoke stack.
It has been proposed heretofore to avoid the reheating of the flue gases, especially in fossil-fuel power plants, by feeding the cleaned flue gas into the cooling air stream which rises in the cooling tower generally provided in such plants, at a location above the heat exchange zone within the cooling tower.
The rising cooling air thus effectively becomes a transport medium for the flue gases. Because the force which causes the rising column of cooling air is a thermally created force other than that which may require a reheating of the flue gas, the expensive reheating step can be avoided.
Indeed, in newly constructed power plants operating in accordance with this principal, a smoke stack or chimney for discharge of the flue gases can be dispensed with altogether.
A further advantage of this process is that the flue gases before they reach the atmosphere are additionally diluted by the transport medium, i.e. the cooling gas flow so that residual toxic concentrations upon release into the atmosphere can be markedly reduced. Of course, a precondition for this method and the improvements which it entails is that the flue gas introduced into the rising cooling air stream will be distributed in this cooling air stream and is uniformly mixed therewith.
In a conventional apparatus which operates in this manner, the flue gas is introduced into the cooling air stream of the cooling tower via a gas outlet duct which opens in a chimney-like fashion, i.e. freely, above the heat exchange zone or baffle region of the cooling tower and which is provided with a central flue gas feed.
Power plants of medium dimensions and outputs generally have up to 600 kg/hour of scrubbing water entrained with the wet-cleaned flue gas in spite of the fact that generally at the outlet of the flue gas desulfurizing unit or scrubber, a droplet or mist collector is provided.
When the desulfurizing scrubber for the flue gas is operated with limestone as an absorbent, the solids content of the entrained scrubbing water can be up to about 20% by weight. As a consequence, the flue gas which is discharged into the cooling tower and through the cooling tower into the atmosphere can entrain up to 120 kg/hour of solids with the entrained scrubbing water.
At a quite normal operating period of about 8000 hrs per year, these figures mean that substantially 960 metric tons of solids are released into the atmosphere and/or deposit in the feedlines to the flue gas outlet duct and, in the latter case can block or plug up the apparatus.
When the cooling tower itself is provided with mist or droplet collectors or traps, deposits of these solids can develop on the surfaces thereof to create detrimental obstructions.
As a consequence, the duty cycle of the apparatus can be adversely effected by the times required for cleaning the surfaces, heat exchange and mist or droplet collection efficiency may be reduced to the detriment of the environment, and it may be necessary to interrupt the operation or effect cleaning during intervals of high electric output, thereby further contributing to the decreased effectiveness of the plant.